Upgrade from TW Acustic Raven AC-3 to what?

Dear Aoliverio, while I will not question the ranking you have listed, I would nevertheless like to mention, that it too depends on the transmission (belt, string, tape - whatever). Introducing a low grip string/thread with a certain "amount" of "wanted slip" then the first group with high mass platter/high inertia will display an extremely smooth and constant speed with little to no measurable derivation once 33 1/3 are obtained. The higher the inertia/mass and the lower the grip of the thread the more constant the speed. I would call this the "Micro-Seiki-RX-Principle" as it first was introduced and widely used by Micro Seiki with their RX-series of turntables. Furthermore the thread with low tension/grip is about the very best "isolator" from motor/flywheel generated vibrations.
As for the motors - in the set-up with low tension thread and high inertia their importance is decreasing. I simply choose the best capstan motors and stop worrying about the issue at all. An expensive, heavy and uninspired choice, but a very satisfying and final one too.
However - the more rigid the coupling between platter and motor and the less mass/inertia in the platter the more importance in the speed stability and low vibration of the motor drive.
Here again I would choose a really good big tape machine capstan - for the very same reason as before: stability of speed, very little vibration.
What we find on our records was recorded with these capstans in big Amperex and Studer machines for 4 decades.
I think if it is good enough for the source of our records, it is good enough for their reproduction.

Dertonarm

You make a very interesting point which may help explain some observations I had with a previous system.

In a way I was thinking about low torque as a way of minimizing a motor speed instability on the platter. I think you are right that some wanted belt slippage would accomplish the same thing. Possibly better.

Couple of comments. Would the belt slippage concept work the same with a DC motor? If these are constant tourque, would the motor try try to compensate for slippage and therefore lose speed? Whereas an AC motor being constant speed would not change pulley speed if the friction changed due to slippage.

Any thoughts?

DT, you wrote,"Now - if you jump from firm solid rock ground it will be different as if you'd jump from a suspended wooden floor and the landing will be different too - right? Do I need to say more ?" You might say more about the relevance of this metaphorical and rhetorical question to turntables. But don't bother on my account.

Aoliviero, There is no free lunch. Each of the common mechanisms for spinning a platter has its particular set of compromises, including high mass platter/loose belt types. If you really want to learn more about it, go to Vinyl Asylum and search for the posts of Mark Kelly on this topic. He has done real experiments, and he can back up his conclusions with physics. Don't get nervous, DT, he likes much about belt drive turntables, but not the whole enchilada.

Aoliviero,

You said...

"I have been pondering the technical aspects behind TT speed stability in a qualitative way. I think there might be situations where mass plays an important role. I tend to think about this as AC vs DC, low vs high torque, low vs high platter Inertia (driven by mass). I tend to bracket in the following fashion:

1) Optimum

a) low torque, high platter inertia, AC
b) low torque, high platter inertia, DC

2) Moderately optimum
a) high torque, high platter inertia, AC
b) high torque, high platter inertia, DC

3) Not so optimum

c) high torque, low platter inertia, AC
d) high torque, low platter inertia, DC
e) low torque, low platter inertia, AC
f) low torque, low platter inertia, DC..."

I agree, but believe the absolute details of it are somewhat determined by the drive implementation and other factors that may be specific to a particular design.

Win
Saskia Turntables

Dear Lewm, well - as I mentioned before - there are many audiophiles and "audio scientists" out there who do favor direct drive or idler drive in turntables ( both principles do build on the idea of "control" and the motor and its quality has a huge impact on the result) and who will present wonderful technical descriptions and "proofs" for their preference.
But that is not my problem. I do use thread/string drive with very low grip and "wanted slippage" to accomplish what I want and use extreme high mass and inertia to get to a kind of self-stabilizing system. This has one huge drawback: - a fairly long time to get to the needed speed.
I am using a motor which is extremely expensive (and weights 12 lbs without any cover ... raw) and would by the way qualify easily for the most demanding ID or DD TTs.
I have told you in a direct email what are the points behind direct drive and idler wheel drive and their origins and original purpose.
We will see all 3 drive principles in various versions side by side for the next decades. Every one of them will have their cheerleaders and followers.
Fine.
No worries Lewm - I certainly will not get nervous about anyone's experiments. I have done my own. Too many people do misinterpret their dreams, philosophy or ideas for physical facts. And any experiments result is depending on its conductors ability (or his will...) to read it and draw the (sometimes unpleasant...) objective conclusions.